1. Field of the Invention
The present invention relates to a method of operating a solid polymer electrolyte fuel cell for aging of the solid polymer electrolyte fuel cell. The solid polymer electrolyte fuel cell includes a membrane electrode assembly comprising a pair of electrodes and an electrolyte membrane interposed between the electrodes. Further, the present invention relates to an aging apparatus for aging of such a solid polymer electrolyte fuel cell.
2. Description of the Related Art
Fuel cells are systems for obtaining direct current electrical energy by supplying a fuel gas (chiefly containing hydrogen) and an oxygen-containing gas (chiefly containing oxygen) to an anode and a cathode for inducing electrochemical reactions at the anode and the cathode.
For example, a solid polymer electrolyte fuel cell includes a power generation cell formed by sandwiching a membrane electrode assembly between separators. The membrane electrode assembly includes the anode, and the cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. In use of this type of the power generation cell, generally, predetermined numbers of the membrane electrode assemblies and separators are stacked together to form a fuel cell stack.
In the solid polymer electrolyte fuel cell of this type, since the amount of water contained in the newly-assembled electrolyte membrane is not sufficient, the initial power generation performance is low. Therefore, in general, in order to achieve the desired power generation performance, aging operation for the fuel cell is performed after the fuel cell has been assembled.
For example, in a method of operating a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2003-217622, in order to cause flooding in the unit cells of the fuel cell at the time of preliminary operation (aging operation) of the fuel cell, the gas utilization ratio of the consumed gas is improved.
However, in the above operating method, since flooding is induced quickly, the control for suppressing degradation in the cell performance is complicated. In particular, the performance of the electrolyte membrane of the MEA may be affected undesirably.
In the case of, e.g., using hydrocarbon material instead of fluorine material for the electrolyte membrane of the MEA, since the hydrocarbon material is hydrophobic in comparison with the fluorine material, it takes a considerable time to impregnate the electrolyte membrane with water.
In an attempt to address the problem, Japanese Laid-Open Patent Publication No. 2007-066666 discloses an aging apparatus for a solid polymer electrolyte fuel cell, and the aging apparatus includes a load consuming load current from the solid polymer electrolyte fuel cell at the time of preliminary operation, and control means connected between the solid polymer electrolyte fuel cell and the load for changing the magnitude of the load current cyclically over time.
According to the disclosure, in the structure, since the magnitude of the load current is changed cyclically over time, water impregnation in the MEA is facilitated, and reduction in the time required for aging operation is achieved.
In Japanese Laid-Open Patent Publication No. 2007-066666, the aging operation is started by supplying a cathode gas to the cathode, supplying an anode gas to the anode, and applying load current from the fuel cell stack to the load such that the magnitude of the load current is changed cyclically over time.
However, when the MEA is used for the first time after it is assembled, power generation cannot be performed at high current density. Therefore, operation of gradually increasing the amount of the applied electrical current from low current density, and operation of reducing the holding time during applying the electrical current to the load and returning the applied voltage to OCV (open circuit voltage) are required.
Under the circumstances, it takes considerable time to achieve the maximum power generation performance of the fuel cell, and such an aging operation is time consuming. Further, since the cathode gas and the anode gas are consumed in the aging operation, in particular, the amount of hydrogen consumption is excessively large. Therefore, the aging operation is extremely uneconomical.